1. Field of the Invention
This invention relates to auto-focus control for use in a video camera and, more particularly, to improvements in producing an auto-focus control signal even if the scene being imaged by the camera is of relatively low contrast or contains areas of high brightness, either of which could impart errors in a focus detection signal.
2. Discussion of the Prior Art
One example of an auto-focus control arrangement is described in copending application Ser. No. 393,804, filed Aug. 15, 1989, now U.S. Pat. No. 4,998,162 and assigned to the assignee of the present invention. In that arrangement, when a camera is focused properly on an object, that is, when the adjustable lens elements included therein are at the "just-focused" position, the image of the object which is focused is provided with sharp boundaries between areas or relative to, for example, the background which is not focused. The video signal derived from this image includes sudden transitions at these sharp boundaries, resulting in high frequency components. Thus, the level of these high frequency components is indicative of the focus condition of the lens. That is, the high frequency components are at a maximum amplitude when the lens is at its just-focused position; whereas the amplitude of the high frequency components is reduced when the lens is at other positions.
As disclosed in the aforementioned application, the higher frequency components (e. g. above 1 MHz and, preferably, above 500 KHz) included in the video signal are integrated; and the value of the integrated signal is used as an indication of the focus condition of the lens. Since the amplitude of the higher frequency components may vary over a scan line and throughout a video field, it is preferred to integrate the higher frequency components to minimize rapid variations in the focus condition. Since a focus condition indication is used to drive a lens adjustment motor, the slowly varying signal produced by integration avoids sudden and constant changes in the motor drive signal.
While the aforementioned auto-focus technique operates generally satisfactorily, errors may be introduced into the focus control signal when the camera images a scene having relatively low contrast, such as a scene that appears as a relatively simple pattern. The video signal produced in response to such an imaged scene has higher frequency components of a relatively low amplitude. However, noise signals that may be passed by the filtering circuitry may be of a sufficiently high amplitude as to be construed erroneously as the higher frequency video signals. Consequently, the noise signals are integrated and used as an indication of the lens focus condition.
Another drawback associated with the aforementioned auto-focus arrangement is present when the scene being imaged contains a portion with a very high brightness level. This may occur if the imaged scene contains an object of high reflectivity, such as a mirror, a polished white subject, or the like, a light source, such as a fluorescent lamp, a flame, a strobe light, the sun, etc. The high brightness portion of the imaged scene produces a video signal whose higher frequency components may be of an excessively high amplitude. Hence, when the higher frequency components are integrated by the focus detection arrangement, the excessive amplitudes caused by the high brightness portion of the imaged scene may produce false indications of the lens focus condition. Additionally, to accommodate these very high amplitudes requires a very wide dynamic range which, in turn, is relatively expensive to implement.